This invention pertains to cementitious construction units of relatively light weight and high strength.
Production of precast concrete beams, slabs and blocks is common. Development of special materials and techniques have greatly increased the range of useful applications of such beams, slabs and blocks and allowed use of smaller, lighter members than previously possible. Still, weight of concrete members has remained a major area of concern to engineers.
Efforts to reduce the weight of concrete units for particular applications have been directed in two general directions. One way has been to reduce the weight of concrete members by casting shapes, for instance, having a Tee, I, channel or hollow cross section. These units can be said to represent rectangular members from which some of the concrete has been taken out in order to save weight. Because the concrete which is left out is in low efficiency areas, the weight decrease is proportionally greater than the strength decrease. Therefore, a lighter member can be made to serve a particular purpose. However, there are limitations to the thinness of the solid portions of the cross section with the methods of manufacture currently in use because of the low tensile strength of plain concrete and the difficulty of placing it in thin-walled monolithic sections. In addition, disruption of strength and integrity occurs from concrete settlement and void form movements while thin concrete sections cast monolithically are hardening.
Another means of producing members by selective placement of concrete in thin sections is extrusion. In this method limitations arise because of support requirements for the thin sections while they are still plastic. In order to keep the product from slumping it is necessary to limit thinness and to utilize rigid trays, forms or storage racks during the curing period.
The second general direction of the efforts to reduce the weight of concrete members has been to use lower density concrete. A decrease in the density of the concrete can be achieved by the utilization of a foaming process or by mixing with very low density fillers. With either method the effect is to lower the strength and modulus of elasticity in essentially direct proportion to the density below a certain point. This technique amounts to indiscriminate removal of solid material from the cross section of the unit, that is, from areas of high structural efficiency as well as from the less efficient areas.
The ability of a structural member to withstand bending and compression column forces is dependent on the size and shape of its cross section. Assuming that the material properties are constant, the strength of a member will vary in proportion to the area, moment of inertia and section modulus of its cross section. Also, for any given cross-sectional area and loading pattern there are certain loci of points which represent the most structurally advantageous positions for placement of the solid material. For instance, in the case of a long member designed to resist bending about a particular axis, as in the case of a beam or wall section, the advantageous loci of points are lines parallel to the axis, located on each side of the axis and as far removed from the axis as the design limitations of the cross section will allow. Thus, the most advantageous use of any material is accomplished by locating as much material as near as possible to the lines previously described. The portions of the unit located near these lines are generally referred to as flanges. The thickness of the flanges required for a particular unit depends on the properties of the material used as well as the forces to be resisted.
In addition, a member must be able to resist shearing forces so as to maintain the relative position of the flanges and to cause the flanges to act integrally as a unit rather than individually. This is accomplished by connecting the flanges with webs, usually lying perpendicular to the flanges and connected to them with sufficient strength to resist the shearing forces.
The most usual cross-sectional shapes which are formed by combination of flanges and webs to take advantage of these principles are the I and the hollow box. The latter can be equated to two I sections joined side by side. However, other structurally advantageous shapes can be made by various combinations of webs and flanges. The basic principle for achieving an optimum combination of strength and lightness is to make members with a cross section comprising voids and solid portions wherein the solids are placed in accurate relation to one another so as to occupy the most structurally advantageous positions in the unit, that is, to serve as the flange and web portions of the unit as described above.
If long members with thin walls are produced by casting monolithically in forms or by extrusion, handling and storage problems while the cemented material is still plastic are considerable. Furthermore, the thin walls will likely be fragile. However, thin sections of excellent integrity, strength and uniformity can be preformed as sheets or slabs in the flat position. An example is asbestos cement sheet which is commonly manufactured with as little as one-eighth inch thickness. Reinforcing and strengthening by addition of fibers or polymers are processes which can be easily employed to best advantage in the flat position. For instance, orientation of fibers is possible by combing and the surfaces are fully available for control of fiber protrusion. Application and penetration of monomer are facilitated, and polymerization by either heat or radiation can be accomplished throughout the depth of the sheet or slab with precision and efficiency not possible otherwise. Additionally, forming cemented material into flat strips or slabs does not require expensive molds and the necessary storage area is minimized because the sheets or slabs can be laid on top of one another without any space between them. Finally, after hardening and curing, the flat sheets or slabs can be throughly inspected, and tested, if desired, to ascertain their suitability for structural purposes. Thus, in a system employing precast or preformed strips or slabs in the manufacture of construction units, the strength and integrity of thin walls could be completely assured prior to fabrication.
A considerable advantage of my invention is that it is feasible to deploy basic precast or precut materials and subassemblies for final fabrication at or near point of use. The sub-assemblies could include precut strips or sheets of cemented material, bags of premixed mortar for the continuous prisms and prefabricated reinforcing harnesses. The major component, the rigid strips or slabs of preformed cemented material, could be packaged very compactly and could be easily protected against damage when shipped in stacks. At the point of use, a fabrication facility to perform the simple operations for unit assembly and extrusion or placement of the continuous bonding prisms would require little investment in plant facilities and relatively small numbers of skilled laborers. These advantages make the invention particularly appropriate for use at remote military installations, construction facilities, town developments and self-help housing projects in developing regions.